A comparison of our results with literature data

A comparison of our results with literature data on the action of pregnane and androstane steroids on the functional activity of NMDAR, GABAAR and GlyR enables us to draw the following conclusions. First, androstane steroids (with no substituents at C-17) at a concentration up to 50 μM do not affect the IGABA and IGly of rat hippocampal pyramidal neurons. At the same time, electrophysiological experiments on NMDAR expressed in HEK293 cells show an ability of these steroids to inhibit NMDAR-mediated currents at low micromolar concentrations (Adla et al., 2017). Therefore, androstane steroids could be considered to be selective NMDAR antagonists. Second, such pregnane steroids as PA-Glu, PA-hSuc, and PA-hPim affected the IGABA and IGly of rat hippocampal neurons with IC50 values in the range of 3–12 μM, and these values are close to previously published steroid effects on the NMDAR current (Vyklicky et al., 2016). Thus, these new pregnane steroids cannot be considered to be selective NMDAR antagonists.
Acknowledgments This work was supported by the Russian Foundation for Basic Research [grant number 16-04-00205], The Russian Science Foundation [grant number 16-15-00235], Czech Academy of Sciences [grant number RVO 61388963], and the Technology Agency of the Czech Republic [grant number TE01020028]. The authors would like to thank Mr. Ben Watson-Jones M. Eng. for language correction.
Introduction One of the most important causes of neuronal hyperexcitability in epilepsy are disturbances in the equilibrium between inhibitory and excitatory systems. It seems probable that a dysfunction of the GABAergic system, which plays an important role in inhibiting neuronal activity, may consequently lower the seizure threshold [1]. For many years, in studies of the mechanisms involved in epileptogenesis, scientists have focused on the role of the activity of the GABA-A receptors localized in the synaptic cleft that mediate phasic inhibition. That type of inhibition is generated by the activation of postsynaptic GABA-A receptors following an Amsacrine hydrochloride [2]. Nevertheless, it seems that another type of GABA-mediated inhibition, called tonic inhibition, may be at least equally important in the regulation of neuronal activity [3]. The involvement of GABA-A receptors in phasic or tonic inhibition is related to their localization, which is determined by the subunit composition of the receptors. The most prevalent subunit compositions of the GABA-A receptors, accounting for approximately 40% of all expressed GABA-A receptors in the brain, include the α1, β2 and γ2 subunits [4]. The presence of the γ2 subunit promotes the synaptic localization of GABA-A receptors [5]. It was demonstrated that GABA-A receptors that contained the γ2 subunit in association with an α1, α2 or α3 and with a β2 or β3 subunit represent the main group of the GABA-A receptors responsible for synaptic, phasic inhibition [3]. The combination of γ2 subunit with the α4, α5 or α6 subunits constitutes the GABA-A receptors that are localized outside the synapse [6]. The GABA-A receptors with an α4, α5 or α6 subunit are responsible for tonic inhibition [3]. Nevertheless, the most important subunit of the GABA-A receptor responsible for tonic inhibition is the δ subunit, which is detected almost exclusively in the extrasynaptically localized GABA-A receptors [7]. The inhibitory function of the GABA-A receptors is also linked to the activity of GABA transporters. There are two main types of those transporters, GAT-1 and GAT-3. GAT-1 is responsible mainly for the neuronal uptake of GABA, while GAT-3 provides the glial uptake [8].
Materials and methods
Results
Discussion The results of our study indicated that in the PTZ kindling model of epilepsy, seizure development changed the hippocampal expression of the GABA-A receptor subunits, including those involved in the mediation of tonic inhibition. We observed a 20% decrease in expression of the δ subunit of the GABA-A receptors which are localized almost entirely at extrasynaptic or perisynaptic sites (e.g., in the cerebellum or dentate gyrus) and mediate tonic inhibition [3].